Supplementary Material. Analog Memory Capacitor Based on Field Configurable Ion- Doped Polymers

Transcription

1 Supplementary Material Analog Memory Capacitor Based on Field Configurable Ion- Doped Polymers By Qianxi Lai et al. 1. X-ray photoelectron spectroscopy (XPS) measurements XPS was used to investigate the concentration of the ions inside the polymer layer in the memory capacitors with the Al/Ti/RbAg 4 I 5 /MEH-PPV/SiO 2 /p-si/al structure after the devices experienced various gate biases. In order to improve the relative resolution of the chemical profiles, the thickness of the MEH-PPV polymer layer in the memory capacitor was increased to 200 nm for the XPS experiments, and the voltages applied on the devices were increased accordingly. Voltage biases V = -8 V and +8 V, were applied to the top Al/Ti electrode with respect to the bottom Al electrode of two devices on the same wafer for 1000 s. The top metal electrodes were then glued to a glass slide with epoxy, and the Al/Ti/RbAg 4 I 5 /MEH-PPV top layers were physically peeled off the SiO 2 /p-si substrate. The chemical depth profiles in the detached Al/Ti/RbAg 4 I 5 /MEH- PPV top layers were subsequently analyzed by XPS by ion-milling from the exposed MEH-PPV side (Fig. S1).

2 FIG. S1. The schematics showing the process to peel off the Al/Ti/RbAg 4 I 5 /MEH-PPV top layers of a memory capacitor, and the subsequent ion-milling process of the polymer layer during XPS analysis. The specimens were analyzed by XPS using a PHI Quantera Scanning ESCA. The spectrometer uses a monochromatic aluminum x-ray source with a photon energy of ev. The analysis was performed using a 200 µm photon beam. The sample was sputtered at a nominal rate of 4.7 nm/min using a 2 kv Ar+ ion beam with azimuthal rotation of the sample. The binding energies were charge corrected to the carbon 1s peak at ev. An example XPS survey spectrum was collected from the MEH-PPV polymer surface in the detached Al/Ti/RbAg 4 I 5 /MEH-PPV top layers after the memory capacitor experienced a voltage bias -6 V, and is shown as a function of the binding energy in Fig. S2. The XPS spectrum indicates the presence of I and Ag ions at the MEH- PPV/SiO 2 interface after the negative voltage bias.

3 Intensity (x10 4 arb. unit) OKLL -OKLL -I3p1 -I3p3 -O1s -I3d3 -I3d5 -Ag3p3 -C1s -Ag3d5 -N1s -Rb3s -Rb3p1 Rb3p3 -Si2s Rb3d -Si2d -I4d -Ag4d Binding Energy (ev) FIG. S2. An XPS survey spectrum collected from an exposed MEH-PPV polymer surface after the Al/Ti/RbAg 4 I 5 /MEH-PPV top layers were detached from a memory capacitor. The chemical depth profiles of I and Ag ions in the polymer layer of the memory capacitor that experienced various voltage biases are shown in Fig. S3. The XPS results indicate that only I and Ag concentrations in the polymer layers were modified significantly by the voltage biases; no perceptible modifications of the other elements have been observed. As shown in Fig. S3, -8 V induced a significant increase of I concentration in the polymer and +8 V induced a decrease of I concentration. This indicates that I anions were driven into the polymer by the negative voltage biases from the RbAg 4 I 5 toward the MEH-PPV/SiO 2 interface, and remained in the polymer after the voltage biases were removed. Surprisingly, the negative voltage biases also induced an increase of Ag concentration in the polymer. It could be inferred that after the negative voltage biases were removed, some Ag cations drifted back into the polymer, attracted by the field induced by the relatively immobile I anions, in order to reduce the net charge in the device.

4 FIG. S3. XPS chemical depth profiles in the MEH-PPV layer of the Al/Ti/RbAg4I5/MEH-PPV/SiO2/p-Si memory capacitor experienced +8 V and -8 V biases. The MEH-PPV/RbAg 4 I 5 interfacial position is roughly estimated by the dramatic increase of the Ag and I concentrations. The inset is the detailed of the Ag and I composition in the MEH-PPV layer. 2. Retention time measurements for control devices The retention characteristics of control devices with the Al/Ti/RbAg 4 I 5 /SiO 2 /p-si, Al/Ti/RbAg 4 I 5 /PEO/SiO 2 /p-si, Al/Ti/RbAg 4 I 5 /PMMA/SiO 2 /p-si, and Al/Ti/MEH- PPV/SiO 2 /p-si structures were measured under the same experimental conditions described in the text in comparison with the Al/Ti/RbAg 4 I 5 /MEH-PPV/SiO 2 /p-si/al memory capacitor. The devices were configured by applying 3 V or -3 V voltage biases, respectively, for 100 s, and then their differential capacitances were tested as a function of time at a temperature of 25 C and a frequency of 1 MHz with a zero DC bias. As shown in Fig. S4, after the capacitances of control devices were configured to different values by the voltage biases, the capacitances of the Al/Ti/RbAg 4 I 5 /SiO 2 /p-si and

5 Al/Ti/RbAg 4 I 5 /PEO/SiO 2 /p-si devices gradually evolved toward quasi-equilibrium values within ~10 3 s, and the capacitances of the Al/Ti/RbAg 4 I 5 /PMMA/SiO 2 /p-si device gradually evolved toward a quasi-equilibrium value within ~10 4 s. After the configurations, the Al/Ti/MEH-PPV/SiO 2 /p-si device quickly evolved toward a quasiequilibrium value within ~10 2 s, and the configured capacitances show negligible difference at the beginning of the measurement at the zero DC bias. The retention times of these control devices were significantly shorter than those observed in the memory capacitor. After a negative voltage bias is applied to the control capacitors with the Al/Ti/RbAg 4 I 5 /PMMA/SiO 2 /p-si and Al/Ti/RbAg 4 I 5 /PEO/SiO 2 /p-si structures, the I anions are driven from the RbAg 4 I 5 layer into the polymer layer. After the negative voltage is removed, attracted by the relatively immobile I anions in the polymer, the mobile Ag cations drift into the polymer, and I anions and Ag cations then form ionic dipoles in the polymer, increasing the polymer permittivity and device capacitance. The weak bonding between the ions and the PMMA or PEO polymer matrixes cannot hold the ions in the polymer, therefore the ions gradually approach their quasi-equilibrium profiles, and the device capacitance gradually evolves toward its quasi-equilibrium value. When a positive voltage bias is applied to the control capacitors, the Ag cations are driven from the RbAg 4 I 5 layer into the polymer layer, and the residual I anions in the polymer are driven toward to the RbAg 4 I 5 layer. After the positive voltage is removed, attracted by the field induced by the relatively immobile I anions, the mobile Ag cations drift toward the RbAg 4 I 5 layer, and the ionic dipoles in the polymer and the device

6 capacitance are reduced. When the ions gradually approach their quasi-equilibrium profiles, the device capacitance gradually evolves toward its quasi-equilibrium value. When a positive or negative voltage bias is applied to the control capacitor with the Al/Ti/RbAg 4 I 5 /SiO 2 /p-si structure, the mobile Ag anions are driven by the electrical field, and accumulated near the RbAg 4 I 5 /SiO 2 or Al/Ti/RbAg 4 I 5 interfaces. The separated ionic charges generate ionic dipoles in the RbAg 4 I 5 layer, increasing the device capacitance. After the voltage bias is removed, attracted by the relatively immobile I anions in the RbAg 4 I 5 layer, the mobile Ag cations recombine with the I anions in the RbAg 4 I 5 layer, and the ionic dipoles in the device and the device capacitance are gradually decreased toward their quasi-equilibrium values. When a positive or negative voltage bias is applied to the control capacitor with the Al/Ti/MEH-PPV/SiO 2 /p-si structure, the capacitor is charged by electrons and holes like the conventional capacitors, no significant ionic charges are involved in the charge process. After the voltage bias is removed, the electrons and holes are discharged quickly at a speed much higher than the ions.

7 (a) Capacitance (nf/cm 2 ) Al/Ti/RbAg 4 I 5 /SiO 2 /Si/Al (b) Capacitance (nf/cm 2 ) Al/RbAg 4 I 5 /PEO/SiO 2 /Si/Al Time (s) Time (s) (c) 18 (d) 40 Capacitance (nf/cm 2 ) Al/Ti/RbAg 4 I 5 /PMMA/SiO 2 /Si Capacitance (µf/cm 2 ) Al/Ti/MEH-PPV/SiO 2 /Si Time (s) Time (ms) FIG. S4. The capacitances of control devices with (a) Al/Ti/RbAg 4 I 5 /SiO 2 /p-si, (b) Al/Ti/RbAg 4 I 5 //PEO/SiO 2 /p-si, (c) Al/Ti/RbAg 4 I 5 /PMMA/SiO 2 /p-si and (d) Al/Ti/MEH- PPV/SiO 2 /p-si structures are displayed as a function of time after the device capacitances were configured by applying -3 V (red line) or +3 V (black line) voltage biases, respectively, for 100 s.